1. Field of the Invention
The present invention relates to a method for producing a silicon nitride filter suitable for removing dust, etc. contained in a high temperature exhaust gas.
2. Description of the Background
Silicon nitride has excellent characteristics with respect to heat resistance, corrosion resistance, chemical resistance, strength, etc., and is expected to be useful as a filter for dust arresting or dust removing in a high temperature or corrosive environment, or as a filter for cleaning the exhaust gas from a diesel engine. Methods for producing such silicon nitride filters have been proposed.
For example, JP-A-6-256069 proposes a method which comprises mixing coarse silicon nitride particles and a glass powder, followed by molding and baking. However, such a method wherein a low melting point material is added as a binder, has a problem that the heat resistance of silicon nitride will be substantially impaired. Further, JP-A-7-187845 and JP-A-8-59364 propose methods wherein a mixture of silicon nitride particles and an organic silicon compound, and a mixture of silicon nitride particles and a polysilazane, are used, respectively, as starting materials, and molded products are likewise baked. However, an organic silicon compound such as a polysilazane is usually expensive, and the methods of using such expensive materials have a problem with respect to the production cost and the availability of the starting materials.
On the other hand, as a method for obtaining a silicon nitride filter by using inexpensive metal silicon particles instead of silicon nitride particles and carrying out nitriding treatment, JP-A-1-188479 proposes a method to obtain a filter having a nitriding ratio of silicon nitride particles of at most 50%, by using as a starting material, a mixed powder comprising metal silicon particles and silicon nitride particles. However, in this method, the nitriding ratio of the silicon nitride particles is at most 50%, whereby there will be a substantial amount of metal silicon particles remaining in the silicon nitride sintered product without being nitrided, whereby the excellent heat resistance and corrosion resistance of silicon nitride are likely to be impaired.